1. Field of the Invention
The present invention relates to a lens array element that is mainly used for a projection type image display device and the like, and particularly relates to a lens array element that has an improved reference surface for being positioned with respect to another optical element, and a projection type display device using the same.
2. Description of Related Art
Projection-type image display devices such as projectors are provided with image display elements, such as a liquid crystal panel or a DMD (digital mirror device). Light from a light source such as a lamp is condensed on the image display element, and spatial modulation is performed thereon according to an input image signal. The optical image formed by performing spatial modulation is magnified and projected, thus enabling large screen display.
With such a projection-type image display device, it is important to make a light beam that illuminates the image display element uniformly in its cross section. This is for efficiently transmitting light emitted from the light source to the image display element so as to reduce the optical loss in an optical system of an illumination device, and also for increasing the in-plane uniformity of an image that is magnified and projected onto a screen.
A lens array element that is a plate-like glass optical element is used for the purposes of making an illumination light beam uniform. FIG. 8 shows an example of an optical system for making an illumination light beam uniform. This optical system is an example in which light emitting diodes are used as light sources. Light emitted from light emitting diodes 21 to 23 is first collimated by lenses 24 to 26. The collimated wide light beams are color-combined by a three-color combining prism constituted from prisms 27 to 29 and optical thin films 30 to 31.
The color-combined light is divided by a lens array portion 32 in which a plurality of cellular lenses are disposed in the same plane. The divided light passes through lenses 33 and 34 and a polarization beam splitter 35, and the divided light beams are superimposed on a reflection-type image display element 36, illuminating the image display element 36. The light modulated and reflected by the image display element 36 is projected onto a screen (not shown) by a projection unit 37.
The lens array portion 32 includes a first lens array element 38 disposed on the light source side, and a second lens array element 39 disposed on the side of an object to be illuminated. Each of the lenses constituting the first lens array element 38 has a figure similar to that of the object to be illuminated. Light beams divided by the individual lenses of the first lens array 38 are focused on the image display element 36, while being superimposed, by a corresponding number of divided lenses constituting the second lens array 39. Accordingly, the image display element 36 can be uniformly illuminated.
When the lens array element is installed in the projection-type image display device, it is necessary to position accurately a light transmission surface, which is an optical function surface of the lens array element, with respect to the optical axis of the optical system. If there is displacement between the center of the lens array element and the optical axis, or angular displacement of the lens array element in the horizontal/vertical directions with respect to the optical axis, an accurate optical path cannot be obtained. As a result, the light utilization efficiency is decreased significantly, and a projection image becomes darker. Also, problems such as the occurrence of illuminance unevenness arise.
In order to install a rectangular plate-like lens array element while being accurately aligned with respect to the optical axis, in other words, another optical element, a method of using the end face of the lens array element as a reference surface has been known. When the end face is used as a reference surface, the accuracy of the shape of the end face of the lens array element is significantly important.
JP 2005-107410A discloses a configuration in which, in order to use the end face of a lens array element as a reference surface for alignment, reference surfaces perpendicular to a plate surface are formed on two adjacent end faces of the lens array element using a metal mold. A region other than the reference surface on the end face forms an inclined surface for providing a draft that inclines slightly inward toward the plate surface on the side on which the cellular lenses are formed.
According to such a configuration, accurate positioning using a reference surface can be performed. Specifically, alignment can be easily performed using both of a side corresponding to the vertical direction of the lens array element and a side corresponding to the horizontal direction thereof and, accordingly, it is possible to prevent angle displacement of the lens array element in the horizontal/vertical directions. Since the end face region other than the reference surface forms the inclined surface for providing a draft, it is easy to release the element from the mold after molding. On the other hand, since the reference surface is perpendicular to the plate surface, highly accurate positioning is possible.
Incidentally, the usage of a projection-type image display device has expanded from various business uses for presentations to home use. Along with this, there has been demand for the miniaturization of such apparatuses, so that a demand for the miniaturization and high accuracy of optical components also has become strong.
In the case of forming a lens array element using a metal mold, in order to satisfy demand for a strictly accurate shape, the pressure applied at the time of molding must be set high. Accordingly, it is difficult to release the molded element from the mold unless sufficient draft is provided.
Accordingly, in the configuration disclosed in JP 2005-107410A, only the reference surface is formed so as to be perpendicular to the plate surface so that alignment accuracy is secured, and by forming the other portion of the end face to be an inclined surface for serving as a draft, the releasing properties in a forming step are secured.
However, with such a configuration, a level difference is formed in a boundary portion between the perpendicular reference surface and the inclined surface. Due to this level difference, when releasing the element from the mold, the boundary portion, in other words the side ends of the reference surface region, may be deformed undesirably. Such deformation causes a decline in alignment accuracy.